There has been proposed a method of forming a surface having an unevenness by pressing parts of a surface of a glass substrate to form a compressed layer and subsequently etching the surface (JP 2002-160943A). With this method, depressions and projections are formed on the surface of the glass substrate due to an etching rate difference between the compressed portion (layer) and the remaining portion within the glass substrate. Use of this method makes it possible to impart minute projections and depressions over a wide surface area far more easily and more efficiently than using a photolithography technique.
However, there is a limitation on the types of substrates to which this method can be applied. The reason is that it is necessary for this method to form within a substrate a compressed layer whose etching rate is reduced to such a degree that projections can be formed. In JP 2002-160943A, a glass substrate containing SiO2 and Al2O3 is disclosed as a substrate suitable for the above-mentioned method.